In 3GPP Long-Term Evolution (LTE) networks, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, e.g., evolved Node-Bs (eNBs) communicating with a plurality of mobile stations referred as user equipments (UEs). Radio link monitoring (RLM) is a mechanism for a UE to monitor the quality of a downlink (DL) for determining if the radio link is good enough to continue transmission. In addition to RLM, it has been proposed that a UE shall consider radio link failure (RLF) to be detected based on at least three problems. First, physical layer problems based on N310/N311/T310 mechanism (DL radio link failure). Second, random access problems indicated from MAC layer (UL radio link failure). Third, problems indicated from RLC layer that the maximum number of retransmission has been reached (UL radio link failure).
A connection failure typically indicates a too late handover, a too early handover, or handover to a wrong cell. In a too late HO problem, a connection failure occurs in the source cell before the handover was initiated or during a handover. The UE attempts to re-establish the radio link connection in the target cell (if handover was initiated) or in a cell that is not the source cell (if handover was not initiated). In a too early HO problem, a connection failure occurs shortly after a successful handover from a source cell to a target cell or during a handover. The UE attempts to re-establish the radio link connection in the source cell. In a wrong cell HO problem, a connection failure occurs shortly after a successful handover from a source cell to a target cell or during a handover. The UE attempts to re-establish the radio link connection in a cell other than the source cell and the target cell.
“RLF report” is used in LTE systems allowing UE to report information to the network about a connection failure event, radio link failure (RLF) or handover failure (HOF). The information includes available geographical location information and available mobility measurement results. The current RLF report is used to optimize handover parameters and to detect coverage problems. For example, eNodeB may utilize information contained in the RLF report to optimize UE measurement configuration and HO algorithm behavior to find acceptable or as low as possible HO problem rate, as well as to find a balanced Ping-Pong rate or HO rate.
A major complication, however, is that handover performance can be very different depending on the UE state and configuration, which may be deliberate and in turn depending on quality of service requirements of the current applications of the UE. The UE state and configuration is not reported in the current RLF report. In many cases, e.g., when the UE goes to idle because of connection failure, it is impossible by the network to correlate the RLF report with configuration information used for the UE by the network node that was serving the UE at the time of failure. In addition, UE internal state like speed state, UL SRB transmission state is not reliably known in the network. Even though a certain amount of UEs report of connection problem events to the network, it may be difficult or impossible for the network to determine if there is a problem that needs corrective action or not. This is because UEs of different state and configuration could be expected to have different performance targets and this UE state or configuration information is not always known by the network.
It is an objective of the current invention to address the shortcomings of prior art by adding sufficient UE state and configuration information to the “RLF report”. It is an objective of the current invention to provide more highly reliable information than a network solution could provide. This is to enable the network to assess what should be the appropriate expected performance targets for a UE reporting on a connection failure event. The information can help the network to determine whether to apply corrective actions to mitigate this kind of failure. Possible corrective actions could involve using less DRX so UE will perform radio measurements with better accuracy, adjusting RLF reporting criteria so that measurement report is sent sooner or later, enhancing transmission for UL/DL by adjusting power control or by using inter cell interference coordination.